1. Field of the Invention
The present invention relates to a wavelength reading apparatus for use in dispersive type monochromators and more particularly, it relates to a monochromator wavelength reading apparatus incorporated with a wavelength counter which is linked with a dispersion element driver unit to read and indicated digitally tuning wavelengths of the monochromator and which is convenient for automatically calibrating read-out values to indicate correct wavelengths.
2. Description of the Prior Art
Conventionally, it has been usual to indicate the tuning wavelength of a monochromator by means of a graduated disc or a mechanical counter mechanically linked with a dispersion element driver unit which is provided to rotate a dispersion element such as a diffraction-grating or a prism. With the tuning wavelength indication of the graduated disc type, however, since the tuning wavelength indicator is mechanically linked with the dispersion element driver unit, the relative position between them is mechanically fixed and in the event of erroneous indications of the indicator, troublesome adjustment for calibration of the tuning wavelength indications are needed. Also, the mechanical counter whose operation speed is limited because of its mechanical structure restricts the increase of the wavelength scanning speed of the monochromator. In addition, those conventional tuning wavelength indicators are usually applicable to a mere indication of the tuning wavelengths of the monochromator and it is very difficult to use them for producing a signal indicative of the tuning wavelength which may be utilized for other apparatus. On the other hand, the digital process technique is becoming popular today and there is a requirement for converting the tuning wavelengths of the monochromator into electrical signals in order, for example, to process the waveform indicative of spectra obtained from a sample analysis utilizing a monochromator, or to apply the spectra obtained from a monochromator to arithmetic operation by a minicomputer which is connected on line to the monochromator. Further, by converting the tuning wavelengths of the monochromator into electrical signals, these electrical signals can be used as wavelength-axis sweep signals of a recorder for recording spectra; as, in the course of the wavelength scanning operation, and used for calibration of wavelength indications, which is carried out at a predetermined wavelength. Further, by using those electrical signals the interchange of light sources, diffraction gratings or detectors and the insertion of a filter into the light path can be effected accurately and automatically.
Because of such advantages as above, in place of the aforementioned graduated disc and mechanical count indicator, one type of wavelength signal generator wherein a potentiometer is linked with the dispersion element driver unit such that a voltage output corresponding to the tuning wavelength of the monochromator is derived out of the potentiometer or another type wherein an encoder is linked with the wavelength driver unit such that a digital output corresponding to the tuning wavelength is derived out of the encoder has been proposed.
These conventional measures, however, are disadvantageous in some aspects. In the first place, with an analog wavelength signal generator utilizing a potentiometer or the like, accuracy and linearity are degraded. For example, in spite of the fact that an accuracy of 4.times.10.sup.-5 is required for converting a wavelength lying in the range of 2500 nm covering ultraviolet, visible and near infrared regions into an electrical signal with an accuracy of 0.1 nm, the potentiometer can provide at the most an accuracy of several to several tens when applied to the wavelength range of 2500 nm. On the other hand, by using two or three digital encoders each having 2 to 3 digits combined with each other through gears, a wavelength count indicator can easily be realized which has much higher accuracy than the analog type indicator. However, the encoder is very expensive and the indicator incorporated with 2 or 3 encoders in combination suffers from a drastically high cost.
In the second place, in case of a monochromator of the type in which two or more diffraction-gratings having different wavelength ranges are used interchangeably in order to provide a wide wavelength range to be scanned, troublesome calculations are required for converting wavelength signal outputs into tuning wavelengths of the monochromator when the diffraction-gratings are changed.
In the third place, a complicated device for the wavelength calibration of the monochromator is required. If the tuning wavelength of the monochromator deviates from the indicated wavelength obtained from the output signal of the wavelength signal generator, which deviation would stem from wear or plays occurring in a rotation mechanism provided in the wavelength driver unit to rotate the dispersion element and a linking mechanism between the wavelength driver unit and the wavelength signal generator, those deviations should be immediately corrected. For correction of such deviations, a wavelength deviation is measured by using a light source having known line spectra and a quantity corresponding to the wavelength deviation is added to or substracted from the output signal of the wavelength signal generator. This requires a special operational or calculation unit.